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What did we go looking for to explain this expression used to transcribe a state of stupefaction, which dates from the end of the 19th century!
We had recourse to the sense of coin that the flaon or flan can have (which would explain why we stay there, minted like money), we saw there a typographer's term, an erroneous spelling of the flanks-buttocks .
Above all, this largely dominates the image of the flan-patisserie, which is prepared with ingredients within everyone's reach, which could be obtained cheaply, for two cents, for two rounds, as Duneton rightly points out, what was said at that time in popular language.
And wouldn't we find there confirmation even in the slight tremor that makes the blank move, like when we accuse the shock of a violent surprise?